1. Field of the Invention
The present invention relates to a vibration detection apparatus for detecting a hand vibration or the like in a camera.
2. Related Background Art
An anti-vibration apparatus for a camera for preventing an image blur caused by a hand vibration or the like by using the above vibration detection apparatus is proposed in U.S. Pat. No. 5,126,561 issued Jun. 30, 1992 or the like
A schematic arrangement of a stabilizing apparatus of this type is shown in FIG. 11.
Referring to FIG. 11, a buoyant member 2 made of a magnetic member supported around a predetermined rotating shaft 3 is arranged in an outer cylinder 1 in which a liquid is sealed. A permanent magnet 4 for maintaining the buoyant member 2 at a reference position is arranged around the outer cylinder 1 so as to constitute a magnetic circuit together with the buoyant member 2. In this state, when the outer cylinder 1 integrally formed with a camera is rotated by .theta.IN by a hand vibration in an absolute space, the buoyant member 2 keeps a stationary state in the absolute space due to the inertia of the liquid in the outer cylinder 1, and the buoyant member 2 is rotated relative to the outer cylinder 1. A rotation amount of the buoyant member 2 from the reference position can be optically detected by a light-emitting element 5 rotated together with the camera and a light-receiving element 6 arranged such that the ratio of its two outputs is changed with a change in light incident position. As a result, an output corresponding to a rotational angle from the reference position is generated by a sensor control circuit 13. Note that elements from the outer cylinder 1 to the light-receiving element 6 and the sensor control circuit 13 correspond to an angular displacement detection apparatus serving as a vibration detection apparatus.
On the other hand, a known variable vertex angle prism 9 is used as an optical means for correcting the image blur. A liquid having a predetermined refractive index is sealed inside the vertical vertex angle prism 9. In addition, as shown in FIG. 11, the vibration detection apparatus is extendible about the rotating shaft shown in FIG. 11. When an incident plane on the object side (right side in FIG. 11) of the variable vertex angle prism 9 is rotated by .theta.OUT with respect to a parallel position, an optical path of a beam incident on a film surface 12 through a photographic lens 11 is proportional to the angle .theta.OUT and at the same time is rotated about an optical axis by an amount corresponding to a proportional constant determined by the refractive index of the liquid.
When an angular displacement in the absolute space is detected by the sensor (i.e., the light-emitting and light-receiving elements 5 and 6), and the vertex angle of the variable vertex angle prism 9 is changed by an amount corresponding to this angular displacement, light from an object is always incident on the same position of the film surface 12, thereby suppressing a hand vibration.
The vertex angle of the variable vertex angle prism 9 is optically detected by a light-emitting element 7 and a light-receiving element 8 in FIG. 11 as in the above angular displacement detection, and as a result, an angular output corresponding to the vertex angle is generated by a position detection circuit 14. Note that an output corresponding to a unit incident angle and generated by the sensor control circuit 13 is equal to an output corresponding to a unit angle of the optical axis and generated by the position detection circuit 14. The outputs from the sensor control circuit 13 and the position detection circuit 14 are input to a subtraction circuit 15 and are subtracted from each other. The difference from the subtraction circuit 15 is amplified by an amplifier 16. The amplified signal is phase-compensated by a phase compensator 17 so as to prevent oscillation of an actuator system loop. In addition, an output from the phase compensator 17 is input to a driver 18. A current is supplied from the driver 18 to an actuator 10 to drive the variable vertex angle prism 9.
With the above arrangement, feedback control is performed such that the output from the sensor control circuit 13 is set equal to that from the position detection circuit 14. In response to a detected hand vibration amount, the photographic optical system can be accurately corrected with respect to the optical axis, i.e., an image blur can be prevented.
In the above conventional arrangement, the buoyant member 2 is kept at the reference position of the outer cylinder 1 by a closed magnetic circuit constituted by the buoyant member 2 and the permanent magnet 4. A force of this closed magnetic circuit acts on the buoyant member 2 as a spring force around the rotating shaft 3, and the characteristics of this force are determined by the magnetic circuit. A vibration stabilizing effect up to a lower frequency level can be obtained when the buoyant member 2 is kept stationary in the absolute space in response to a vibration signal having a low frequency. For this purpose, a magnetic force of the closed magnetic circuit is decreased, and the buoyant member 2 is set not to follow movement of the outer cylinder 1. That is, a spring constant of the spring force must be set small.
When sensor performance is extended to the lower frequency side, rise characteristics are degraded due to a small spring force. For example, when a camera pointing position is abruptly changed as in panning, a large force acts on the sensor itself, and a relative position between the outer cylinder 1 and the buoyant member 2 is greatly changed. A force for returning the buoyant member 2 to the reference position is only the spring force and a viscous force of the liquid. When this spring force is small, a long stabilizing time is required. A vibration stabilizing function does not work until the buoyant member 2 is returned to the reference position. When a photographic operation using a vibration stabilizing function is to be performed, a shutter time lag is undesirably prolonged.